So you are saying that even if 12400F is faster even with just DDR4, you'd choose an AM4 board with DDR4 instead? (I'm hearing a lot of similar reasoning out there....; it's just that I don't understand it.
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For me the best option is Ryzen (Zen3 or Zen3D) since I am on AM4 already.
For a new system, depends. 12400F is not going to be faster than 5600x, it will maybe match it on average (wait for trusted reviews), which will make AMD drop the price, so in that case you won't see neither a difference in performance nor in price (not much anyway).
If AMD does not drop the price, then yes, for a new system you should go for 12400F with DDR4 for a good budget build, but I think 5600x will be $250 (possibly lower or a 5600 for lower price) and then you need to do the math and see CPU+MB+RAM how much of a difference it actually is between them.
Also, take into account, this is very important, your AM4 will also support the upcoming Zen3D, which is due in a few months, not 1 year away like Raptor Lake is, so a fast easy upgrade path compared to Alder Lake.
DDR5 is just for those with more money than sense now, but in 1 year from now, then it will make more sense. This was shown by HUB and GN too, same with Win11. Both are too soon, too little and not worth it.